Update 9 dec '14
Some people mentioned the casing snapped while inserting the gears. I've added new files, including separate STL files with a smaller hinge, so it wond bend as much when assembling. However, cheap and/or old PLA filament will snap very easily when inserting the gears.
The layer bonding of cheap filament is very bad and the material also gets more brittle overtime. As you can see in the picture above, blue has a very good layer bond and I needed quite a lot of force to break it, while the grey one snapped very easily without leaving any residus.